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Overview of shock ignition
Shock Ignition WorkshopMarch 8-10, 2011Rochester, NY
R. BettiFusion Science CenterLaboratory for Laser Energetics
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K. Anderson (LLE)R. Nora (LLE)C. Stoeckl (LLE)W. Theobald (LLE)J. Bates (NRL)A. Schmitt (NRL)M. Lafon (CELIA)X. Ribeyre (CELIA)G. Schurtz (CELIA)S. Weber (CELIA)V. Tykhonchuk (CELIA)S. Atzeni (U. Rome)J. Perkins (LLNL)O. Klimo (CTU)and others……
I will show results from:
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• The physics of shock ignition
• Target optimization
• 1D gain curves
• 2D simulations
• Experimental results
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• Thick shells (with large fuel mass) produce high gains if ignited
• Thick shells have good hydro-stability properties (because they are thick)
• For a fixed laser energy, thick shells have low implosion velocity
• Low implosion velocity leads to low hot spot pressure (P~Vi2-3)
• Low pressure hot spots do no ignite (Pτ> 30 Gbar/ns)
• The energy required for ignition scales as E~ 1/P2-2.5
How do we ignite low-velocity implosions?
The puzzle of high gains: how to ignitelow-velocity imploding targets
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Two different mechanisms related to rarefactionwave propagation limit the hot spot pressure
R. Nora and R. Betti, submitted to Phys. Plasmas
Without rarefaction waves, the peak hot spotpressure would be twice as high
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Withoutrarefaction
Withrarefaction
R. Nora and R. Betti, submitted to Phys. Plasmas
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Late shocks can suppress rarefaction waves.There are three ways shocks can be launched.
No-rarefactiontechnique(requires manyhighly-synchronized“weak” shocks)
No-transmissiontechnique(requires onemoderate shock)
Re-shocktechnique(requires onestrong shock)
R. Nora and R. Betti, submitted to Phys. Plasmas
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The enhancement in hot spot pressure variesfor different techniques (total energy is kept constant)
No-rarefactiontechnique with two“weak” shocks
In principle,using ∞ shocks
No-transmissiontechnique with one shock
Re-shock technique with one shock (for a given shock strength)
R. Nora and R. Betti, submitted to Phys. Plasmas
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The re-shock technique produces the highest hot spot pressure
Shock pressurein Gbar
The shock pressureis high because ofthe planar geometry
R. Nora and R. Betti, submitted to Phys. Plasmas
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In spherical geometry, convergence effects amplifythe ignitor shock by about ten fold
V1
V2
0.5 0.6 0.7 0.8 0.9 10
100000
200000
300000
400000
500000
600000
Cent
ral P
(Mba
r)
Time (ns)
V1
V2
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50
100000
200000
300000
400000
Cent
ral P
(Mba
r)
R
RH
O
P
0 50 100 1500
20
40
60
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100
120
010002000300040005000600070008000900010000110001200013000140001500016000
P(M
bar)
R
RH
O
P
0 50 100 1500
50
100
150
200
0
10000
20000
30000
40000
P(M
bar)
P
ρ
P=7Gbar
P=20Gbar
P
ρ
500Mbar Shock
2Gbar Shock
no shock
w shock
w shock
no shock
~2 pressureenhancement
~3 pressureenhancement
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LILAC agrees with the predictions of the simplemodel for both shock amplification and pressureenhancement
300-400Mb Shock
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Optimal SI targets are wetted-foam shells(in the absence of hot electron pre-heat)
SI wetted-foam target have low IFARsand good stability properties
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Shock-ignition targets with CH ablators have higherIFARs. Hydrodynamic instabilities can be a concern.
K. Anderson (this workshop)
Also see K. Anderson (this workshop)
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Gain curves for shock ignition look impressive but need to assess the sensitivity to preheat (during the main pulse) and (for CH targets) to laser imprinting.
JL Perkins (LLNL) A. Schmitt (NRL)
3-fold pressure amplifications are found for HiPER targets.Gains curves are derived by hydro-equivalent scaling
Lafon, Ribeyre, Schurtz, Phys. Plasmas 17, 052704 (2010)
Ribeyre, Schurtz, Lafon, Galera, Weber, PPCF 51, 015013 (2009)
The spike power and launching time are optimizedfor HiPER shock ignition targets
HiPER shock ignition target
Bates, Schmitt, Fyfe, Obenschain, Zalesak, High Energy Den Phys 6, 128 (2010)
Comprehensive 2D simulations of SI KrF targets, withzooming are carried out by the NRL group
GAIN=103EL = 398kJ
Ignitor-return shock collision seems to reduce thedeceleration RTI growth before ignition
Atzeni, Davies, Hallo, Honrubia, Maire, Olazabal, Feugeas, Ribeyre,Schiavi, Schurtz, Breil, Nicolai, Nucl. Fusion 49, 055008 (2009)
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1D PIC simulations at SI-spike relevant intensities showlow-temperature hot electrons with an energetic tail
Klimo, Weber, Tikhonchuk, Limpouch, Plasma Phys Cont Fus 52, 055013 (2010)
. Thot~ 45keV
C. Stoeckl, APS 2009; W. Theobald et al, PPCF 51, 124052 (2009);
Neutron yields:Compare SI and CHS targets,40 µm CH shells filled with25 atm D2 gas
Areal densities:Compare SI and CHS targetsVarying fill pressures
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Higher neutron yields and areal densities are measuredin shock ignition experiments using thick CH targets
W. Theobald et al, Phys Plasmas 15, 056306 (2008)
Beam pointing schemes are being explored for Polar Drive Shock Ignition on the NIF
• Focusing separate shock beams at a smaller radius late in time allows better coupling of energy to the target.
• A scheme with split quads would allow best irradiation uniformity on target, but requires time-consuming “rewiring” of NIF seed pulses.
• Another scheme employing full quads, half for the main drive and half for the shock pulse was recently proposed* by Steve Craxton
*Craxton, et al., APS-DPP 2010
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• Need to demonstrate the generation of >300Mb shockwaves in long density scalelength plasmas
• Need to demonstrate that hot electrons (mostly from TPD)during the main pulse can be controlled
• Need to demonstrate that the hot electrons above 100keVduring the intensity spike do not preheat the capsule
• Need to demonstrate hot-spot integrity at the highconvergence ratios typical of shock ignition
Significant progress has been made in the past two years, but there are still important issues to be resolved for the validation of shock ignition
Conclusions
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